A typical wireless microphone system of the prior art comprises a transmitter unit which, as shown in FIG. 2, has:
a) a low frequency amplifier 1 for amplifying audio signal output such as from an electric guitar, PA1 b) a signal logarithmic compression circuit 2, (a noise reduction circuit for reducing the effect of noise during transmission), PA1 c) a frequency modulator 3 for modulating the amplitude of the logarithmically compressed signal, PA1 d) multiplier circuits 4, 5 for multiplying the frequency of the modulated signal from the modulator 3 PA1 e) a power amplifier 6 for amplifying the power of the resultant frequency modulated signal, and PA1 f) an antenna 8 for transmitting the amplified signal to a receiver (not shown).
As shown in FIG. 1, the frequency modulator 3 of the prior art has a crystal resonator (Xtal) which oscillates at its natural frequency (fundamental frequency), fo. The frequency fo is supplied to the gate of field-effect transistor F1 of the frequency modulator 3. The frequency modulator 3 is a modification of a fixed quartz oscillator or Sabaroff oscillator, and its drain tank circuit 32 (C tap type) tunes to a frequency which is three times as high as the natural frequency fo. Since the linearity of the FM modulator becomes degraded if the modulation index mf becomes high, the modulation index is controlled to be low. In the multiplying stages, multiplication is carried out so as to obtain a desired modulation index mf. According to the conventional transmitter of FIG. 1, a multiplied (double) frequency is output from the 2.times.multiplier circuit 4. Likewise, another doubling of frequency is output from the 2.times.multiplier circuit 5. Thus, the resulting carrier frequency is 12 times as high as the natural frequency of the crystal Xtal and is supplied to the power amplifier 6.
A transmitter of the above type is typically carried, for example, by a person making a speech to a large audience. Here the transmitter is worn by the speaker and the sound of his voice is transmitted to a receiver. Because portability is important, there is strong demand to make the unit light in weight and compact in size. In addition, if the transmitter unit is installed on a music instrument such as an electric guitar, the price of the unit must be much lower than that of the guitar itself. In order to satisfy these size and cost constraints, the transmitter unit should have as few parts as possible, without degrading its electrical characteristics.